This invention relates to bonded abrasive articles and particularly those bonded with a bond material that can be converted to a semi-crystalline ceramic bond.
A vitreous bonded abrasive product, such as a conventional grinding wheel, comprises three volume components: an abrasive particulate material which usually occupies about 40 to 50 vol. %; a vitreous bond material that provides typically about 5 to 15 vol. % of the total; and the balance of the volume is void space. The function of the bond material is to hold the abrasive particles in place so that they can do the abrading work. In a typical vitreous bonded product of the prior art the glass components are added to the abrasive particles and the mixture is heated till the glass components melt, fuse to form a glass, and then flow to the particle contact points to form a bond post that solidifies on cooling. This provides the rigid structure of the finished product. In a more recent method the glass bond material is formed separately as a molten mass, cooled to solidify and then ground up. This ground up material, known as a frit, is then mixed with the abrasive particles. The advantage of this procedure is that the heating step can be shortened, the bond composition is more uniform and the forming temperature can often be reduced.
It will be appreciated that the rigidity and strength of the products of the prior art are often determined by the bond posts. Glass, being an amorphous material, has a low strength, (about 40 to about 70 MPa), by comparison with the abrasive particles. This low strength gives rise to premature release of grain and enhanced wear. Hence the grinding ability of vitreous bonded products is in theory limited by the strength of the posts. In practice, with most abrasives, such limitations were not very significant. Some more modern abrasives however are adapted to perform best under a heavy load and this puts the bond under considerable stress. Traditional glass bonds are often found inadequate under such conditions and there is therefore a need for vitreous-based bonds with a greater ability to operate under high stresses.
It has been proposed that there might be advantage in the use of a glass-ceramic bond to bond abrasives. However it has not been found possible heretofore to ensure that the bond material is concentrated in the bond posts or in coating the abrasive grits. This of course is extremely inefficient and has not resulted in any commercialization of such glass-ceramic bonded materials in spite of the potential advantages that might be expected.
The present invention provides such a bond material. It has significantly greater strength than traditional bonds and is easily formed. Abrasive products comprising such bond materials often perform substantially better than those made with prior art bonds. The bonds can be used with a wide variety of abrasives and exhibit an impressive versatility in the kinds of abrasive products that can be made with them.